1. Field of Invention
This invention concerns a modulated light wave demodulator for demodulating a light wave, useful for wave-length multioptical communications and coherent optical communications making use of light waves with well-arranged spatial phases, that is to say, coherent light waves.
2. Description of the Prior Art
The coherent optical communication technology which transmits signals through modulating the frequency and phase of a light wave with well-arranged spatial phases, that is to say, coherent light waves, is expected to be the communication technology of the next generation because of its capability for the transmission of a great volume of signals at a long distance. However, no good device, or demodulator has been found for demodulating a modulated light wave. There is a need for a cheaper modulated light wave demodulator with a high accuracy in the wave length of the optical carrier wave and stable tuning capability.
FIG. 4 shows a structure of a conventional modulated light wave demodulator in a current coherent optical communication system. In FIG. 4, element 401 is a modulated light wave; element 402 is an optical fiber to transmit the modulated light wave; element 403 is a light multiplexer; element 404 is a distributed feedback (DFB) semiconductor laser; element 405 is a light mixer composed of PIN photodiodes; element 406 to an intermediate frequency amplifier; element 407 to a filter, and element 408 is to a controller for controlling the semiconductor laser 404.
Next, the operation of this device is described. The laser light wave generated by the semiconductor laser 404 for use as a local oscillator light wave is multiplexed with the modulated light wave 401 carrying information to be transmitted in the form of frequency modulation or phase modulation by the light multiplexer 403. If the wave length of the signal light carrier wave is extremely close to the wave length of the local oscillator light wave, a beat signal is generated in the multiplexed wave. If the beat signal becomes an appropriate intermediate frequency, it is amplified by the intermediate frequency amplifier 406, and thereafter processed in the same way as with ordinary frequency modulation (FM) signals, to demodulate the modulated wave. Tuning is conducted by feeding back the signal obtained through the filter 407 to the controller 408 to control the input current volume of the laser 404. However, in order to increase the accuracy of tuning detection, a stable laser with an extremely narrow spectrum width of its generated light wave length is necessary to use the laser as a light source of local oscillator light waves.
As such, the DFB laser using a waveguide type diffraction grating as the reflector for optical resonance of the semiconductor laser is used. An example for the continuous operation made first at the room temperature was reported by M. Nakamura in Applied Physics Letters, Vol. 27, No. 7, pp 403-405 (1975), "CW operation of distributed-feedback GaAs-GaAlAs diode lasers at temperatures up to 300K".
FIG. 5 shows a structure of the typical conventional DFB laser. In the figure, element 501 is indicates a laser oscillator; element 502 a light waveguide; element 503 is a waveguide type diffraction grating; element 504 is a surface of open section to be the reflector for optical resonance on the laser light emission side, and element 505 is a substrate. The light generated by recombination of electrically injected electrons and holes at the laser oscillator 501 enters the waveguide type diffraction grating 503 through the optical waveguide 502. The entered light interferes with the diffraction grating here, and only the light of a wave length having a certain relationship to the period of the diffraction grating is reflected. The reflected light returns to the optical waveguide portion again, is reflected once more by the ordinary reflector 504 formed on the other end surface of the laser oscillator, and after all, the optical resonator is formed with the waveguide type diffraction grating 503 and the reflector 504. Accordingly, the resonance wave length is fixed with the period of the diffraction grating and stable so that the laser of the narrow spectrum width of its generated light wave length may be obtained. In addition, also known is the external cavity type semiconductor laser for which the ordinary optical diffraction grating is installed outside instead of the waveguide type diffraction grating. This diffraction grating is used as a reflector at one side of optical resonance. A typical example is described by M. Fleming, et al. in "Spectral Characteristics of External-Cavity Controlled Semiconductor Lasers", IEEE Journal of Quantum Electronics, Vol. QE-17, No. 1, pp 44-59 (1981).
There are found some problems in the above-mentioned modulated light demodulator. First, the performance of the semiconductor laser used as a local oscillator is not sufficient. Therefore, it is difficult to tune well to the wave length of the carrier wave of the modulated light. The waveguide type diffraction grating is used for the DFB type semiconductor lasers, but the wavelength selection performance is not sufficient such that the spectrum width of the generated light wave length is not narrow enough. For the external cavity type semiconductor laser, a diffraction grating with a good wavelength selection performance can be used, but it must be fixed mechanically with an accurate optical axis adjustment because of being distant in space, so that it may be difficult to adjust the optical axis precisely, and even if accurately adjusted and mounted once, it is difficult to produce a practically stable laser because of thermal expansion and contraction caused by temperature changes or displacement caused by mechanical vibrations. The spectrum width of the generated light wave length is stable when the spatial distance is lengthened, but in such condition the more likely the above-mentioned mechanical changes takes place. Accordingly, the external cavity type semiconductor laser does not likewise have a sufficient performance.
Secondly, because the wavelength-division-multiplexing communication systems and coherent optical communication systems multiplex light waves, it will be desirable that the wider range of the wave length of the optical carrier wave be available. However, the variable range of the wavelength is narrow in the above-mentioned DFB semiconductor laser. Therefore, the tuning range of the wavelength is narrow.
Furthermore, the use of a tuning filter makes the structure of the device complex.
In the conventional devices as described above, the spectrum width of the generated light wave length is narrow and there is no available semiconductor laser which is stable during changes in the surrounding environmental conditions and with the wide variable range of wavelength, and therefore, no cheap modulated light wave demodulator with a high and stable tuning accuracy and a wide tuning range has been found.